A lab in Kyoto is at the apex of efforts to update traditional Japanese timber construction methods to meet exacting modern standards

For lovers of traditional Japanese architecture, a visit to Akihisa Kitamori’s laboratory at the Kyoto University Research Institute for Sustainable Humanosphere (RISH) would likely evoke similar emotions to those felt by an animal-rights activist in a cosmetics test lab full of tormented rabbits.

In one corner of the large cluttered room, a reproduction of a ceiling-support structure from the main hall of Nara’s Yakushi Temple is clamped to a test rig. As Kitamori turns a knob, more than two tons of pressure bears down on the replica, representing the force that could result from a major earthquake. Gradually, the seven silky-smooth pieces of interlocking wood slip a few centimeters out of place.

In another corner, a grinning lab assistant uses a similar metal rig to apply pressure to a long rectangular post constructed from two pieces of wood that use a traditional tsugite end-joint to increase the length of the timber. Suddenly, after about 10 minutes, the post lets out a series of loud pops as a long black fissure races down its right side.

The tsugite and ceiling-support structure whose strength Kitamori is testing are just two of countless puzzlelike joints Japan’s master carpenters once fashioned to build ancient temples and traditional homes. But while this country’s wood-building heritage is one of the richest in the world, such techniques have been largely replaced by modern alternatives — in part because of earthquake-safety reasons.

“Today building regulations are very strict and it’s difficult to apply traditional techniques to new buildings,” says Kaori Fujita, an associate professor at the University of Tokyo specializing in timber engineering.

As the environmental and aesthetic appeal of traditional wood buildings is now drawing new interest, though, scientists are working to modify age-old techniques to meet today’s safety standards. Kitamori, 31, an earthquake-engineering specialist and assistant professor at RISH’s Laboratory of Structural Function in Uji, Kyoto, is among them.

“The philosophy behind our research at RISH is to create a sustainable society. Wood is a renewable resource — over the 50 or so years a wood house is used, new timber can be grown, and that creates a sustainable cycle,” says Kitamori, who works with the lab’s head professor, Kohei Komatsu, and assistant professor, Tokuro Mori, to test and improve wood joints.

Kitamori adds that Japan’s extensive and underutilized timber plantations mean wood is abundantly available here — though for economic reasons, more than three-quarters of the timber used is currently imported.

Today, about 86 percent of new detached houses in Japan have wood frames, according to government statistics. However, most are built using contemporary techniques that gain strength and stiffness from diagonal wall braces, lots of nails and screws, and walls made of plywood or other paneling.

In contrast, the traditional construction methods that Kitamori and his lab-mates study feature intricately joined timbers and earth walls held together without diagonal braces or metal fasteners. This style of building was common from the 12th through 19th centuries, and can still be seen in many temples and shrines.

But while many such large structures have survived centuries of quakes, houses built using the same techniques on a smaller scale do not always perform as well. During the disastrous 1995 Kobe earthquake in central Japan, for example, many traditional wood houses collapsed — although Fujita queries whether the problem was structure or old age.

“We can make very strong buildings using traditional techniques, or we can make them very weak. The problem is not in the system itself, but in the dimensions of the structural elements, how they are connected, and their maintenance,” she says. “In traditional structures, larger columns are usually much safer.”

Large timbers are scarce in Japan today, however, and that’s where Kitamori’s research comes in.

“I want to take the best and strongest elements from traditional houses and incorporate them into modern ones,” he says.

His research focuses on improving the strength of traditional walls, which are built around a frame of horizontal beams called nuki that pass through vertical posts. Frequently, each intersection of post and beam is kept rigid by inserting a small wooden wedge called a kusabi into it. Traditionally, a bamboo lattice was then added and covered with a mixture of mud and straw to form a tsuchikabe, or earth wall.

The strength and stiffness of walls (both internal and external) is a crucial element in Japan’s current seismic safety assessments, which impact building permits, loans and home insurance. Each type of wall is assigned a number rating its ability to resist pressure from earthquakes and wind. This number varies based on the construction method and materials.

Traditional earth walls have the lowest rating, just one third that of walls reinforced with a diagonal brace 3 cm thick and 9 cm wide.

Kitamori says the reason the rating is so low is that without nails to keep everything in place, traditional walls move back and forth during minor tremors, though they often remain standing in stronger quakes. That’s because wood inevitably shrinks over time, so small gaps develop in the joints — which translates into wiggle room during a temblor.

Unlike in olden times, stiffness in response to small quakes is important in contemporary houses, because even small movements can damage windows, pipes and other fixtures.

To address that problem, Kitamori is experimenting with wedges made of compressed wood to secure the joints where horizontal and vertical timbers intersect. The wood, which is compressed using moderate heat and high pressure, expands in response to humidity so the wedges gradually swell over time to fill the gaps in the joint. Kitamori uses average- quality sugi (Cryptomeria japonica) for the wedges and wall framework because that is the cheapest and most abundant type of wood grown in Japan today.

According to research he presented at the 2008 World Conference on Timber Engineering held in Miyazaki, Kyushu, pressurized wood wedges were found to nearly triple the stiffness of joints in tests simulating a minor earthquake.

But why the focus on expensive and labor-intensive all-wood carpentry when there are cheaper ways to build strong wood houses using nails and two-by-four timbers? For Kitamori, the answer involves a blend of practical, cultural, and environmental factors.

“Prefab houses where the wood is hidden behind plywood and walls are hard to fix, and it’s impossible to see rot or damage from termites,” he says. In contrast, timbers are usually exposed in traditional architecture.

The thousands of nails, screws, and other metal fixtures used in a modern wood house pose environmental problems as well. Since manufacturing metal is an energy-intensive process, these fixtures add to a house’s carbon footprint. They’re also a problem when it comes to demolishing a house, since wood and metal must be separated for recycling or reuse. And other materials used in modern houses, such as glue, paint and plastic, can create unhealthy fumes.

But for Kitamori, the draw of traditional homes goes deeper than carbon calculations and termite troubles.

“I think part of the reason people are taking another look at traditional houses is that (compared to conventional modern houses) they feel more like they’re made of wood, and people love that,” he says. “They’re not robotlike houses chock full of nails and screws, but rather houses made by human hands. That’s our tradition, and so I think even if it takes a little more effort, building traditional-style houses is a way to get people to fall in love with their homes and live in them for a long time.”

In a country where the average house becomes landfill after just 30 years, that would perhaps be the greatest environmental accomplishment of all.